To use the Agda2 Emacs mode:

0. Install haskell-mode, version 2.1 or later (see
   http://haskell.org/haskell-mode/).

0.1 Add

    (add-to-list 'load-path "<The Directory Where You Downloaded haskell-mode>")

    to your .emacs file.

1. Check out and compile Agda2.

   Make sure that you install the GHC package; the binary is not used
   by the Emacs mode.

   For details, see somewhere else.

2. Copy the .el files in src/full/Interaction/emacs-mode to some directory
   which is listed in your Emacs load-path. You may want to
   byte-compile the files.

   Untested example installation (creates a new directory and adds it
   to your load-path, does not byte-compile the files):

   mkdir <...>/agda2-mode
   cp src/full/Interaction/emacs-mode/*.el <...>/agda2-mode
   echo "(add-to-list 'load-path \"<...>/agda2-mode/\")" >> ~/.emacs

3. Add the following to your .emacs

   (autoload 'agda2-mode "agda2-mode" "Agda2 mode." t)
   (add-to-list 'auto-mode-alist '("\\.l?agda$" . agda2-mode))

   If you have already set up the suffix .agda for use with some other
   mode you may need to remove or alter that setting. You are
   encouraged to use the suffix .alfa for Agda 1 files.

4. If you want to you can customise some settings. Just start Emacs
   and type the following:

   M-x load-library RET agda2-mode RET
   M-x customize-group RET agda2 RET

   This should not be necessary, though.

Now, if you open a file named XXX.agda the buffer will use agda2-mode.
It may take 5 or 10 seconds before you see anything in the buffer, but
that is normal.